Recently, optical communication represented by, e.g., an inter-city network or intercontinental/intracontinental backbone network or an optical interconnection applied to, e.g., a server/router requires an optical pulse train having a high repetition frequency as the communication speed or the capacity increases. An example of an apparatus for generating this optical pulse train is an apparatus using a fiber ring mode-locked laser or mode-locked semiconductor laser.
In a conventionally proposed fiber ring mode-locked laser, a filter region having a ring resonator or two optical waveguide grating routers is formed inside or outside a fiber ring, and a desired high-order longitudinal mode is selectively transmitted (see, e.g., Japanese Patent Laid-Open No. 11-298072, pp. 3-6, FIGS. 2, 3, 10, and 11).
Also, in a conventionally proposed optical clock pulse train generator based on a mode-locked semiconductor laser, an external cavity arrangement includes a semiconductor laser element, a reflecting mirror optically coupled with the semiconductor laser via a collimating lens, and a wavelength selecting element and an etalon formed between the semiconductor laser element and reflecting mirror, the facet of the semiconductor laser element and the reflecting mirror form a main resonator, the etalon functions as a sub resonator having a resonator length that is an integral fraction of that of the main resonator, and a round-trip frequency that is an integral multiple of that of an optical clock pulse train determined by the main resonator is generated (see, e.g., Japanese Patent Laid-Open No. 2003-31897, pp. 3-6, FIGS. 1 to 3).